The broad objective of the proposed research is the elucidation of stimulus determinants and neuroendocrine substrates of sexual behavior in rats. The research is concerned with two main problem areas: 1) neuroendocrine substrates of estrous behavior; and 2) the role of ultrasonic vocalization in the sexual behavior of rats. Research in the first area will be concerned with the localization of sites of action of estradiol and progesterone in the regulation of estrous behavior in female rats. The experiments in this area will employ techniques of intracranial application of steroid hormones, hormone antagonists, and metabolic blockers in order to provide a detailed neuroanatomical identification of structures involved in the mediation of hormonal actions on behavior. The second area involves research on the behavioral correlates of ultrasonic vocalizations, the physical analysis of these sounds, and their precise role in communication in rat sexual behavior.